The invention relates to a cathode-ray tube (CRT), having an internal magnetic shield and, more particularly, to a means for attaching the shield to a frame of a shadow mask.
A color CRT typically has a magnetic shield to reduce the influence of magnetic fields on electron beam trajectories as a cathodoluminescent screen of the tube is scanned. In particular, the angles of incidence of the electron beams at every point on the shadow mask must not change significantly from the design values, or the beams will move away from their intended landing positions on the screen. The magnetic shield may be disposed either outside the tube, as an external magnetic shield, or inside the tube, as an internal magnetic shield.
The internal magnetic shield is usually made of 0.10 to 0.18 mm thick cold-rolled steel, and is fastened to a shadow mask frame by resilient clamping pins which are inserted through aligned apertures disposed in the shield and the frame. The frame is supported by springs that engage mounting studs that extend inwardly from a rectangular glass faceplate panel of the tube. During tube fabrication, the internal magnetic shield is fastened to the frame, by the clamping pins, prior to the step of frit sealing a sidewall of the faceplate panel to a glass funnel of the CRT. The internal magnetic shield is designed to fit into the funnel and to be as close to the funnel wall as possible. However, it should not touch the funnel, in order to avoid any friction between the shield and a conductive anode coating on the inner surface of the glass funnel. Additionally, the shield must be attached securely to the frame, without any significant gaps therebetween, to ensure good magnetic coupling and to prevent overscanned electrons of the beams from passing between the shield and the frame and being scattered to the screen.
One drawback of the clamping pins is that their use requires careful alignment of the corresponding apertures formed in the shield and the frame. Typically, the pins are inserted by hand, thus adding additional labor and cost to the manufacture of the CRT. Also, improperly inserted pins may loosen or become dislodged during handling of the tube, resulting in a change in location of the shield and a resultant change in its magnetic shielding properties, causing misregister of the electron beams with the phosphor elements of the screen. In extreme cases where a pin becomes dislodged, the loose pin may cause electrical failure of the CRT.
A need therefore exists for an improved means of attaching the internal magnetic shield to the shadow mask frame to avoid the above-described problems associated with clamping pins.